Exhaust gas recirculation (EGR) systems recirculate a portion of exhaust gas from an engine exhaust to an engine intake system to improve fuel economy and vehicle emissions by reducing throttling losses and combustion temperatures. In turbocharged engines, an EGR system may include a low-pressure EGR (LP-EGR) circuit that diverts exhaust gases from downstream of a turbine of a turbocharger and injects the gases before a compressor. However, the LP-EGR circuit has a long transport delay, as the exhaust gases must travel through the turbocharger compressor, high pressure air induction plumbing, charge air cooler, and intake manifold before reaching the combustion chamber. As a result, it may be difficult to provide a desired amount of EGR to the cylinders, particularly during transient conditions.
One example approach for managing the long transport delay is shown by Styles et al. in US 20120023937. Therein, the LP-EGR system is operated at a fixed EGR percentage rate of fresh air flow across an area of a speed-load map, including a minimum engine load in order to improve transient control of LP-EGR (e.g., during a driver tip-out event when minimum load may be encountered).
However, the inventors herein have recognized issues with the above approach. Specifically, in an engine configured for variable selective cylinder deactivation, when transitioning between operating modes, due to differences in EGR tolerance, engine misfires can occur. As such, the fixed EGR percentage is based on an EGR tolerance level during the minimum engine load. Herein, engine load, or load, will be used to describe the overall engine air flow or engine torque. Cylinder load will be used to describe an average air flow per active cylinder in the engine. In this way, cylinder load will increase at the same engine load when cylinders have been selectively deactivated. However, in engines configured for selective cylinder deactivation (e.g., variable displacement engines (VDE)), the EGR tolerance level of the engine may vary based on whether the engine is operating with all cylinders active or with one or more cylinders deactivated. For example, when operating in a VDE mode with one or more cylinders deactivated, due to an increased cylinder load on the remaining active cylinders, a minimum cylinder load encountered in the VDE mode may be greater than the minimum cylinder load during a non-VDE mode when all the cylinders are active and combusting. Consequently, the engine may tolerate higher EGR levels when operating in the VDE mode than when operating in the non-VDE mode. Therefore, if the fixed EGR percentage is based on engine operation in the VDE mode, the EGR percentage may be greater than required for the non-VDE mode. As a result, there may be excess dilution of intake air in the non-VDE mode which may increase combustion stability issues and the propensity for engine misfires.
Further, during a tip-out occurring when operating in the VDE mode with the fixed EGR percentage based on the VDE minimum load, due to large transport delays associated with the LP-EGR system, the engine operation may transition out of the VDE mode (in order to reduce Noise Vibration and Harshness (NVH) issues, for example) before the EGR is purged from the air induction system. As a result, the engine may be exposed to higher EGR levels than is tolerable by the engine in the non-VDE mode, leading to increased combustion instability and misfires.
In one example, some of the above issues can be at least partly addressed by a method for an engine comprising: in response to a tip-out occurring while operating the engine below a threshold engine load with one or more cylinders deactivated and with LP-EGR provided at a higher fixed schedule relative to intake air flow, delaying reactivation of the deactivated cylinders until EGR has reduced from the higher fixed schedule to a lower fixed schedule relative to intake air flow. In this way, cylinder reactivation may be adjusted based on cylinder EGR tolerance to reduce occurrence of misfire events.
As an example, a VDE engine system may include a LP-EGR system for providing EGR. When operating the VDE engine in a VDE mode with one or more cylinders deactivated and in a speed-load range for fixed EGR percentage, EGR may be provided at a higher fixed EGR percentage due to the higher EGR tolerance of the remaining active cylinders that are operating at a higher average cylinder load. In comparison, when operating in a non-VDE mode with all the cylinders active, EGR may be provided at a lower fixed percentage due to the lower EGR tolerance of engine cylinders operating at a lower average cylinder load. Further, during a tip-out condition occurring when operating in the VDE mode, it may be desirable to transition out of the VDE mode in order to reduce NVH issues or due to hardware constraints, for example. During such tip-out conditions when engine transition from VDE mode to non-VDE mode is desired, if an engine load is below a threshold engine load, transition of engine operation out of the VDE mode may be delayed until the EGR percentage relative to intake air flow decreases to the lower fixed percentage. This avoids the condition where the engine is operating with a higher EGR schedule than the engine can tolerate. However, if the engine is operating in the VDE mode at an engine load above the threshold load, the engine may be allowed to transition out of the VDE mode while transitioning EGR from the higher fixed percentage to the lower fixed percentage.
In this way, by providing EGR in two different fixed schedules including the higher fixed percentage of EGR during the VDE mode and the lower fixed percentage of EGR during the non-VDE mode, during each mode, the engine may be operated with an EGR schedule that the engine can tolerate, while allowing the EGR schedule to be varied based on the different EGR tolerances in the different modes. By allowing a higher flat EGR schedule to be applied when selected cylinders are deactivated, the EGR percentage in the air induction system can be raised, increasing the effectiveness of EGR. By allowing a lower flat EGR schedule to be applied during the non-VDE mode, excess intake air dilution with EGR may be reduced, reducing slow burn issues. Further, by delaying engine transition out of the VDE mode responsive to a tip-out until the EGR level corresponding to the VDE mode is cleared out of the air induction system, engine misfires and slow combustion issues due to engine exposure to higher than tolerable EGR levels is reduced. Consequently, engine misfire events are reduced and combustion stability may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.